The present invention relates generally to both gas turbines and steam turbines, and more particularly to turbine blade cover arrangements in turbine blades. Turbine blade covers are typically cooled to decrease the bulk temperature and maximum temperature of the blade cover material to support higher mechanical loads. Cooling the turbine blade cover affords the designer the ability to improve the aerodynamic performance configuration of the turbine blade cover.
Steam turbines include, but are not limited, to steam turbine power generation equipment and shipboard steam turbine propulsion equipment. Gas turbines include, but are not limited to, gas turbine power generation equipment and gas turbine aircraft engines. An exemplary steam turbine typically contains a high-pressure turbine section, a low-pressure turbine section, or a combination of both, which is rotated by the steam flow. An exemplary gas turbine typically includes a core engine, having a high pressure compressor to compress the air flow entering the core engine, a combustor in which a mixture of fuel and the compressed air is burned to generate a propulsive gas flow, and a high pressure turbine which is rotated by the propulsive gas flow and which is connected by a larger diameter shaft to drive the high pressure compressor. A typical front fan gas turbine aircraft engine adds a low pressure turbine (located aft of the high pressure turbine) connected by a smaller diameter coaxial shaft to drive the front fan (located forward of the high pressure compressor) and to drive an optional low pressure compressor (located between the front fan and the high pressure compressor). The low-pressure compressor sometimes is called a booster compressor or simply a booster.
In the exemplary gas turbine, typically the fan and the high and low pressure compressors and turbines have gas turbine blades each including an airfoil portion attached to a shank portion. In the exemplary steam turbine, typically the high and low pressure turbine sections have steam turbine blades each including an airfoil portion attached to a shank portion. Rotor blades are gas or steam turbine blades attached to a rotating gas or steam turbine rotor discs, respectively. Stator vanes are gas turbine blades or steam turbine blades attached to a non-rotating gas or steam turbine stator casings, respectively. Typically, there are alternating circumferential rows of radially-outwardly extending rotor blades and radially-inwardly extending stator vanes. When present in the gas turbine configuration, at least one of a first and a last row of stator vanes (also called inlet and outlet guide vanes) may have their radially-inward ends also attached to a non-rotating gas turbine stator casing. Counter rotating “stator” vanes are also known in gas turbine designs. Conventional gas and steam turbine blade designs typically have airfoil portions that are made entirely of metal, such as titanium, or are made entirely of a composite. The all-metal blades, including costly wide-chord hollow blades, are heavier in weight, resulting in lower fuel performance and requiring sturdier blade attachments. The blades typically have a blade cover that extends over the top of the airfoil portion of the turbine blade. The blade cover typically provides rigidity to individual blades by covering a plurality of adjacent blades. The blade cover is typically configured with aerodynamic features to minimize stage to stage air or steam leakage in the turbine.
In a gas turbine aircraft application, the gas turbine blades that operate in the hot gas path are exposed to some of the highest temperatures in the gas turbine. Various design schemes have been pursed to increase the longevity of the blades in the hot gas path. By way of example and not limitation, these design schemes include blade coatings, internal cooling of the blades, and internal cooling of the blade covers.
In one common internal core cooling arrangement, a series of radial cooling holes extend through the entire turbine blade. The turbine blade is first manufactured as a solid blade. The solid blade is then drilled using Electro-Chemical Machining (ECM) or Shaped-Tube Electro-Chemical Machining (STEM), to create a plurality of through holes from about a blade root to about a blade tip. The radial cooling holes in axially long blades can be difficult to machine, sometimes requiring drilling from both ends of the blade. The blade with the radial cooling holes tends to have more mass than is desired. The extra mass can be problematic during thermal transients as the interior surfaces and the exterior surfaces of the blade do not respond at the same rate to the thermal transient, which results in thermal stresses. Moreover, the use of radial cooling holes is generally not possible in the leading and trailing edges of the blades, due to the three dimensional curvature of the blade. The blade cover typically has radially drilled cooling holes that overlie the radial cooling holes in the blades after the blade cover is disposed over the blade. The blade cover also typically has drilled passages that direct the coolant to the edges of the blade cover to eject the coolant to the working fluid in the turbine. The coolant absorbs heat from the blade cover to decrease the bulk temperature and maximum temperature of the blade cover material.
The coolant for the internal cooling of the blades and the blade covers typically comes from a cooler temperature part of the turbine or from a separate source of cooling. The coolant is typically either an air-based coolant or a steambased coolant. The air-based coolant is typically bled either from the compressor section or from a post-compressor region that surrounds the combustion section that is operating at a cooler temperature than the turbine blades and blade covers of concern. The air-based coolant is alternately supplied from a separate off-machine located air supply system. The steam-based coolant is typically supplied from a turbine section that is operating at a cooler temperature than the turbine blades and turbine blade covers of concern. Alternatively, the steam-based coolant is supplied from an independent steam supply (i.e. other steam system or auxiliary boiler). However, providing the coolant to internally cool the turbine blades and turbine blade covers represents internal work to the turbine that reduces an efficiency of the turbine. Additionally, the issues related to directing the flow of the coolant to the areas of highest heat load in the blade cover has created the desire to improve the internal cooling of the blades and blade covers even further.
Accordingly, there is a need for an improved turbine blade cover. What is needed is a blade cover cooling apparatus that allows more aggressively shaped aerodynamic configurations to reduce stage-to-stage leakage and delivers higher cooling effectiveness. What is also needed is a blade cover cooling apparatus cooling scheme that satisfies the blade cover cooling requirements with less impact on the turbine net output.